Stereo echo suppressing device, echo suppressing device, stereo echo suppressing method, and non transitory computer-readable recording medium storing stereo echo suppressing program

ABSTRACT

According to the embodiment of the present invention, the stereo echo suppressing device that suppresses an acoustic echo signal based on a near-end input signal and far-end signals of two channels as a stereo signal includes: first and second far-end signal amplitude spectrum calculating units for calculating amplitude spectrums of a first far-end signal and a second far-end signal; a near-end input signal amplitude spectrum calculating unit for calculating an amplitude spectrum of the near-end input signal; first and second estimation echo signal estimating units for estimating amplitude spectrums of a first estimation echo signal and a second estimation echo signal; first and second echo suppressing units for suppressing the acoustic echo signal; first and second echo path characteristic updating unit for calculating and updating echo path characteristics of the first far-end signal and the second far-end signal.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is based upon and claims benefit of priority fromJapanese Patent Application No. 2014-173072, filed on Aug. 27, 2014, theentire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to stereo echo suppressing devices, echosuppressing devices, stereo echo suppressing methods, and non-transitorycomputer-readable recording medium storing stereo echo suppressingprogram. For example, the present invention can be applied to a stereoecho suppressing device used for a teleconferencing system and an audioconferencing system.

For example, in an amplifying call system such as the teleconferencingsystem and the audio conferencing system, sounds emitted from aloudspeaker (here, the “sound” includes acoustic, voice, and the like)input to a microphone as acoustic echo signals and return to a talkerside. The acoustic echo signals seriously hinder telephone calls.Therefore, many studies and developments on acoustic echo suppressingmethods have been performed.

One of techniques for suppressing acoustic echo signals includes atechnique using an echo canceller. The echo canceller is a device forsuppressing an acoustic echo signal by estimating echo pathcharacteristics (spatial transfer characteristics) between a loudspeakerand a microphone with an adaptive filter; calculating an estimation echosignal by convolving a far-end output signal with the echo pathcharacteristics estimated; and subtracting the estimation echo signalfrom an near-end input signal.

The echo canceller tends to be composed of a single loudspeaker.However, the teleconferencing system, the audio conferencing system, orthe like each tends to be composed of two loudspeakers so as to outputstereo signals. “ISCIE Journal ‘Systems, Control and Information’, vol.46, No. 12, pp. 724-732 (2002), Stereophonic Acoustic Echo Cancellation:An Overview and Solutions, Shoji MAKINO, and Suehiro SHIMAUCHI”(Non-Patent Literature 1) proposes a stereo echo canceller that causesan echo canceller to be compatible with stereo output and suppressesacoustic echo signals in such case.

SUMMARY

However, by the configuration in which the existing echo canceller iscompatible with stereo output, there is a coefficient indeterminacyproblem that true echo path characteristics are not estimated when thereis a correlation between a Left channel and a Right channel of far-endsignals inputted to the adaptive filter. The coefficient indeterminacyproblem is caused by existence of an infinite number of solutions whenthe adaptive filter is updated so as to reduce errors in an adaptablealgorithm.

Although Non-Patent Literature 1 also discloses a technique for solvingthe problem, a technology described in Non-Patent Literature 1 addsnon-correlated noises and cross-correlation variations to far-endsignals. Therefore, there is a new problem that the far-end signalsthemselves are distorted and deterioration in sound quality andtelephone speech quality occurs.

Thus, use of a stereo echo suppressing device that suppresses stereoechoes in a frequency domain without using the adaptable algorithm isalso proposed. Since the adaptable algorithm is not used in the stereoecho suppression process, there is no coefficient indeterminacy problem.

However, when a signal in which a Left channel and a Right channel of afar-end signal are correlated is inputted, despite almost all acousticecho signals are suppressed in a first suppression process, correlatedacoustic echo signals are also suppressed in a second suppressionprocess. Accordingly, too much suppression (too much subtraction) mayoccur.

Accordingly, there have been desired a stereo echo suppressing device,echo suppressing device, stereo echo suppressing method, and stereo echosuppressing program that prevent too much suppression and suppressacoustic echo signals stably even when a signal in which a Left channeland a Right channel of a far-end signal are correlated is inputted.

Accordingly, the embodiments of the present invention haveconfigurations as described below.

According to a first embodiment of the present invention, there isprovided a stereo echo suppressing device that suppresses an acousticecho signal based on a near-end input signal and far-end signals of twochannels as a stereo signal, the stereo echo suppressing deviceincluding: (1) a first far-end signal amplitude spectrum calculatingunit configured to calculate an amplitude spectrum of a first far-endsignal by transforming the inputted first far-end signal into a spectrumin a frequency domain; (2) a second far-end signal amplitude spectrumcalculating unit configured to calculate an amplitude spectrum of asecond far-end signal by transforming the inputted second far-end signalinto a spectrum in a frequency domain; (3) a near-end input signalamplitude spectrum calculating unit configured to calculate an amplitudespectrum of the near-end input signal by transforming the inputtednear-end input signal into a spectrum in a frequency domain; (4) a firstestimation echo signal estimating unit configured to estimate anamplitude spectrum of a first estimation echo signal by multiplying astored first echo path characteristic by the amplitude spectrum of thefirst far-end signal; (5) a second estimation echo signal estimatingunit configured to estimate an amplitude spectrum of a second estimationecho signal by multiplying a stored second echo path characteristic bythe amplitude spectrum of the second far-end signal; (6) a first echosuppressing unit configured to suppress the acoustic echo signalsuperimposed on the near-end input signal by calculating a first echosuppression gain with the amplitude spectrum of the first estimationecho signal and the amplitude spectrum of the near-end input signal; (7)a second echo suppressing unit configured to suppress the acoustic echosignal superimposed on the near-end input signal by calculating a secondecho suppression gain according to a correlation result between theamplitude spectrum of the first estimation echo signal and the amplitudespectrum of the second estimation echo signal; (8) a first echo pathcharacteristic updating unit configured to calculate and update the echopath characteristic of the first far-end signal by using the amplitudespectrum of the first far-end signal and the amplitude spectrum of thenear-end input signal; and (9) a second echo path characteristicupdating unit configured to calculate and update the echo pathcharacteristic of the second far-end signal by using the amplitudespectrum of the second far-end signal and the amplitude spectrum of thenear-end input signal.

According to a second embodiment of the present invention, there isprovided an echo suppressing device including: one or a plurality of thestereo echo suppressing device as described above, in the echosuppressing device the respective near-end input signals are inputtedfrom one or a plurality of microphones.

According to a third embodiment of the present invention, there isprovided a stereo echo suppressing method of suppressing an acousticecho signal based on a near-end input signal and far-end signals of twochannels as a stereo signal, the stereo echo suppressing methodincluding: (1) calculating an amplitude spectrum of a first far-endsignal by a first far-end signal amplitude spectrum calculating unit, bytransforming the inputted first far-end signal into a spectrum in afrequency domain; (2) calculating an amplitude spectrum of a secondfar-end signal by a second far-end signal amplitude spectrum calculatingunit, by transforming the inputted second far-end signal into a spectrumin a frequency domain; (3) calculating an amplitude spectrum of thenear-end input signal by a near-end input signal amplitude spectrumcalculating unit, by transforming the inputted near-end input signalinto a spectrum in a frequency domain; (4) estimating an amplitudespectrum of a first estimation echo signal by a first estimation echosignal estimating unit, by multiplying a stored echo path characteristicof the first far-end signal by the amplitude spectrum of the firstfar-end signal; (5) estimating an amplitude spectrum of a secondestimation echo signal by a second estimation echo signal estimatingunit, by multiplying a stored echo path characteristic of the secondfar-end signal by the amplitude spectrum of the second far-end signal;(6) suppressing the acoustic echo signal superimposed on the near-endinput signal by a first echo suppressing unit, by calculating a firstecho suppression gain by using the amplitude spectrum of the firstestimation echo signal and the amplitude spectrum of the near-end inputsignal; (7) suppressing the acoustic echo signal superimposed on thenear-end input signal by a second echo suppressing unit, by calculatinga second echo suppression gain according to a correlation result betweenthe amplitude spectrum of the first estimation echo signal and theamplitude spectrum of the second estimation echo signal; (8) calculatingand updating the echo path characteristic of the first far-end signal bya first echo path characteristic updating unit, by using the amplitudespectrum of the first far-end signal and the amplitude spectrum of thenear-end input signal; and (9) calculating and updating the echo pathcharacteristic of the second far-end signal by a second echo pathcharacteristic updating unit, by using the amplitude spectrum of thesecond far-end signal and the amplitude spectrum of the near-end inputsignal.

According to a fourth embodiment of the present invention, there isprovided a non-transitory computer-readable recording medium storing astereo echo suppressing program that suppresses an acoustic echo signalbased on a near-end input signal and far-end signals of two channels asa stereo signal, the stereo echo suppressing program causing a computerto function as: (1) a first far-end signal amplitude spectrumcalculating unit configured to calculate an amplitude spectrum of afirst far-end signal by transforming the inputted first far-end signalinto a spectrum in a frequency domain; (2) a second far-end signalamplitude spectrum calculating unit configured to calculate an amplitudespectrum of a second far-end signal by transforming the inputted secondfar-end signal into a spectrum in a frequency domain; (3) a near-endinput signal amplitude spectrum calculating unit configured to calculatean amplitude spectrum of the near-end input signal by transforming aninputted near-end input signal into a spectrum in a frequency domain;(4) a first estimation echo signal estimating unit configured tocalculate an amplitude spectrum of a first estimation echo signal bymultiplying a stored echo path characteristic of the first far-endsignal by the amplitude spectrum of the first far-end signal; (5) asecond estimation echo signal estimating unit configured to calculate anamplitude spectrum of a second estimation echo signal by multiplying astored echo path characteristic of the second far-end signal by theamplitude spectrum of the second far-end signal; (6) a first echosuppressing unit configured to suppress the acoustic echo signalsuperimposed on the near-end input signal by calculating a first echosuppression gain by using the amplitude spectrum of the first estimationecho signal and the amplitude spectrum of the near-end input signal; (7)a second echo suppressing unit configured to suppress the acoustic echosignal superimposed on the near-end input signal by calculating a secondecho suppression gain according to a correlation result between theamplitude spectrum of the first estimation echo signal and the amplitudespectrum of the second estimation echo signal; (8) a first echo pathcharacteristic updating unit configured to calculate and update the echopath characteristic of the first far-end signal by using the amplitudespectrum of the first far-end signal and the amplitude spectrum of thenear-end input signal; and (9) a second echo path characteristicupdating unit configured to calculate and update the echo pathcharacteristic of the second far-end signal by using the amplitudespectrum of the second far-end signal and the amplitude spectrum of thenear-end input signal.

According to the embodiments of the present invention, it is possible tosuppress acoustic echo signals without occurring the coefficientindeterminacy problem even when a signal in which a Left channel and aRight channel of a far-end signal are correlated is inputted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a stereo echosuppressing device according to a first embodiment;

FIG. 2 is a block diagram showing a configuration of a stereo echosuppressing device according to a second embodiment;

FIG. 3 is a block diagram in a case in which a plurality of the stereoecho suppressing devices according to the first embodiment are mounted;

FIG. 4 is a block diagram in a case in which a plurality of the stereoecho suppressing devices according to the second embodiment are mounted;and

FIG. 5 is a block diagram illustrating an existing echo canceller.

DETAILED DESCRIPTION OF THE EMBODIMENT(S)

Hereinafter, referring to the appended drawings, preferred embodimentsof the present invention will be described in detail. It should be notedthat, in this specification and the appended drawings, structuralelements that have substantially the same function and structure aredenoted with the same reference numerals, and repeated explanationthereof is omitted.

(A) First Embodiment

Hereinafter, details of a stereo echo suppressing device, a stereo echosuppressing method, and a stereo echo suppressing program according tothe first embodiment of the present invention are explained withreference to the drawings.

In the first embodiment, the present embodiment is applied to an echosuppressing device, method, and program, the echo suppressing devicebeing mounted on a voice transmitting and receiving device in theamplifying call system such as the teleconferencing system or the audioconferencing system.

(A-1) Configuration in First Embodiment

FIG. 1 is a block diagram showing a configuration of the echosuppressing device according to the first embodiment.

An echo suppressing device 100 according to the first embodimentincludes two microphones and two loudspeakers, and is mounted on thevoice transmitting and receiving device that outputs stereo signals fromtwo loudspeakers.

In this embodiment, a case in which stereo signals of two channels (forexample, Left channel and Right channel) are inputted to the echosuppressing device 100 as far-end signals, and the two loudspeakersoutput the stereo signals of the two channel is exemplified. However,the number of channels is not limited.

Hereinafter, structural elements that performs a process using one soundsignal among stereo sound signals input to the two microphones aredistinguished by reference signs with “a” at the end, and structuralelements that performs a process using the other sound signal aredistinguished by reference signs with “b” at the end.

With reference to FIG. 1, the echo suppressing device 100 according tothe first embodiment includes far-end signal input terminals 101 a and101 b, digital-analog (DA) converters 102 a and 102 b, loudspeakers 103a and 103 b, microphones 104 a and 104 b, analog-digital (AD) converters105 a and 105 b, stereo echo suppressing devices 100 a and 100 b, andnear-end output terminals 116 a and 116 b.

The stereo echo suppressing devices 100 a and 100 b each include far-endsignal frequency domain transform units 106 a and 106 b, far-end signalamplitude spectrum calculating units 107 a and 107 b, echo pathcharacteristic storage units 108 a and 108 b, estimation echo signalcalculating units 109 a and 109 b, a correlation component rejectingunit 110, a near-end input signal frequency domain transform unit 111, anear-end input signal amplitude spectrum calculating unit 112, echosuppression gain calculating units 113 a and 113 b, echo suppressionprocessing units 114 a and 114 b, a near-end output signal time domaintransform unit 115, echo path characteristic calculating units 117 a and117 b, and echo path characteristic updating units 118 a and 118 b.

Note that, the “first echo suppressing unit” described in claimsincludes the echo suppression gain calculating unit 113 a and the echosuppression processing unit 114 a, and the “second echo suppressingunit” described in claims includes the correlation component rejectingunit 110, the echo suppression gain calculating unit 113 b and the echosuppression processing unit 114 b.

The stereo echo suppressing devices 100 a and 100 b perform stereo echosuppression processes on near-end input signals inputted from therespective microphones 104 a and 104 b. Even when far-end signalstransfer as acoustic echo signals in a space at a near-end side and areinputted to the microphones 104 a and 104 b, the stereo echo suppressingdevices 100 a and 100 b preform the stereo echo suppression processes onnear-end input signals and appropriately suppress the acoustic echosignals.

The stereo echo suppressing devices 100 a and 100 b may be, for example,constructed as dedicated boards, may be implemented by writing echosuppressing programs to digital signal processors (DSPs), or may beimplemented by software (stereo echo suppressing programs) executed byCPUs. Functions of the stereo echo suppressing devices 100 a and 100 bare shown in FIG. 1. All structural elements of the stereo echosuppressing devices 100 a and 100 b may be constructed by hardware, or apart of the structural elements may be constructed by software(program).

Note that, the stereo echo suppressing devices 100 a and 100 b have thesame configurations. Accordingly, hereinafter, only the stereo echosuppressing device 100 a is explained, and repeated explanation of thestereo echo suppressing device 100 b is omitted.

The far-end signal input terminals 101 a and 101 b are connected to, forexample, a network such as an Internet Protocol (IP) network or radiowaves or the like of a wireless network such as a mobile phone. Via aconnected line, far-end signals at a far-end side (partner side) areinputted to the far-end signal input terminals 101 a and 101 b.

Far-end signals inputted to the far-end signal input terminals 101 a and101 b are outputted to the DA converters 102 a and 102 b. The DAconverters 102 a and 102 b convert the far-end signals from digitalsound signals to analog sound signals. Subsequently, the signals areoutputted to a near-end side via the loudspeakers 103 a and 103 b.

On the other hand, the microphones 104 a and 104 b receive analog soundsignals superimposed with environmental sounds at the near-end side,acoustic echo signals (for example, analog sound signals outputted fromthe loudspeakers 103 a and 103 b, transferring in the space at thenear-end side, and inputting) and sound signals such as the talker'sspeech at the near-end side.

The AD converters 105 a and 105 b convert the analog sound signalsprovided from the microphones 104 a and 104 b into digital soundsignals. Subsequently, the digital sound signals are inputted to thestereo echo suppressing device 100 as near-end input signals.

The far-end signal frequency domain transform units 106 a and 106 b eachacquire the far-end signals inputted to the far-end signal inputterminals 101 a and 101 b, transform the respective far-end signals in atime domain into spectrums in a frequency domain by using a fast Fouriertransform (FFT) or the like, and output frequency spectrums of thefar-end signals to the far-end signal amplitude spectrum calculatingunits 107 a and 107 b.

The far-end signal amplitude spectrum calculating units 107 a and 107 bcalculate amplitude spectrums of the far-end signals on the basis of thefrequency spectrums of the far-end signals, and output the calculatedamplitude spectrums of the far-end signals to the estimation echo signalcalculating units 109 a and 109 b and the echo path characteristiccalculating units 117 a and 117 b.

The echo path characteristic storage units 108 a and 108 b store echopath characteristics, and outputs the stored echo path characteristicsto the estimation echo signal calculating units 109 a and 109 b and theecho path characteristic updating units 118 a and 118 b.

The estimation echo signal calculating units 109 a and 109 b calculateamplitude spectrums of estimation echo signals by multiplying the echopath characteristics by the amplitude spectrums of the far-end signals.The estimation echo signal calculating unit 109 a outputs the calculatedamplitude spectrum of the estimation echo signal to the correlationcomponent rejecting unit 110 and the echo suppression gain calculatingunit 113 a. The estimation echo signal calculating unit 109 b outputsthe calculated estimation echo signal to the correlation componentrejecting unit 110.

The correlation component rejecting unit 110 rejects a correlationcomponent of the amplitude spectrum of the estimation echo signal fromthe estimation echo signal calculating unit 109 a included in theamplitude spectrum of the estimation echo signal from the estimationecho signal calculating unit 109 b. The correlation component rejectingunit 110 outputs, to the echo suppression gain calculating unit 113 b,amplitude spectrum of the estimation echo signal from which thecorrelation component has been rejected.

The near-end input signal frequency domain transform unit 111 acquires anear-end input signal outputted from the AD converter 105 a, andconverts the near-end input signal in a time domain into a spectrum in afrequency domain by using the FFT or the like, and outputs the frequencyspectrum of the near-end input signal to the near-end input signalamplitude spectrum calculating unit 112 and the echo suppressionprocessing unit 114 a.

The near-end input signal amplitude spectrum calculating unit 112calculates an amplitude spectrum of the near-end input signal on thebasis of the frequency spectrum of the near-end input signal, andoutputs the calculated amplitude spectrum of the near-end input signalto the echo suppression gain calculating units 113 a and 113 b and theecho path characteristic calculating units 117 a and 117 b.

The echo suppression gain calculating unit 113 a calculates an echosuppression gain for suppressing the acoustic echo signal of theloudspeaker 103 a superimposed on the near-end input signal, by usingthe amplitude spectrum of the near-end input signal from the near-endinput signal amplitude spectrum calculating unit 112 and the amplitudespectrum of the estimation echo signal from the estimation echo signalcalculating unit 109 a. The echo suppression gain calculating unit 113 aoutputs the calculated echo suppression gain to the echo suppressionprocessing unit 114 a.

The echo suppression gain calculating unit 113 b calculates an echosuppression gain for suppressing the acoustic echo signal of theloudspeaker 103 b from which the correlation component has been rejectedsuperimposed on the near-end input signal, by using the amplitudespectrum of the near-end input signal from the near-end input signalamplitude spectrum calculating unit 112 and the amplitude spectrum ofthe estimation echo signal from which the correlation component has beenrejected by the correlation component rejecting unit 110. The echosuppression gain calculating unit 113 b outputs the calculated echosuppression gain to the echo suppression processing unit 114 b.

The echo suppression processing unit 114 a calculates a frequencyspectrum suppressing the acoustic echo signal of the loudspeaker 103 asuperimposed on the near-end input signal by multiplying the echosuppression gain from the echo suppression gain calculating unit 113 aby the frequency spectrum of the near-end input signal from the near-endinput signal frequency domain transform unit 111. The echo suppressionprocessing unit 114 a outputs the frequency spectrum suppressing theacoustic echo signal to the echo suppression processing unit 114 b.

The echo suppression processing unit 114 b calculates a frequencyspectrum suppressing the acoustic echo signal of the loudspeaker 103 bfrom which the correlation component has been rejected superimposed onthe near-end input signal by multiplying the echo suppression gain fromthe echo suppression gain calculating unit 113 b by the frequencyspectrum from the echo suppression processing unit 114 a. The echosuppression processing unit 114 b outputs the frequency spectrumsuppressing the acoustic echo signal to the near-end output signal timedomain transform unit 115, as a frequency spectrum of a near-end outputsignal.

The near-end output signal time domain transform unit 115 transforms thefrequency spectrum of the near-end output signal in a frequency domaininto a digital sound signal in a time domain by using an inverse fastFourier transform (IFFT) or the like, and outputs the near-end outputsignal to the near-end signal output terminal 116.

The near-end signal output terminal 116 is connected to, for example, anetwork such as an IP network or radio waves or the like of a wirelessnetwork such as a mobile phone. Via a connected line, the near-endsignal is outputted to the far-end side.

The echo path characteristic calculating units 117 a and 117 b calculateecho path characteristics of a current frame on the basis of theamplitude spectrums of the far-end signals from the far-end signalamplitude spectrum calculating units 107 a and 107 b and the amplitudespectrum of the near-end input signal from the near-end input signalamplitude spectrum calculating unit 112, and output the calculated echopath characteristics of the current frame to the echo pathcharacteristic updating units 118 a and 118 b.

The echo path characteristic updating units 118 a and 118 b update theecho path characteristics on the basis of the echo path characteristicsof the current frame calculated by the echo path characteristiccalculating units 117 a and 117 b and the echo path characteristicsstored by the echo path characteristic storage units 108 a and 108 b,and the echo path characteristic storage unit 108 a and 108 b store theupdated echo path characteristics.

(A-2) Operation in First Embodiment

Next, details of operation of echo suppression process in the echosuppressing device 100 according to the first embodiment of the presentinvention are explained. The operation is also explained by using thestereo echo suppressing device 100 a as a representative of the stereoecho suppressing devices 100 a and 100 b.

First, after the operation of the echo suppressing device 100 isstarted, far-end signals at the far-end side are inputted to the far-endsignal input terminals 101 a and 101 b via a connected line connected toa network such as an IP network or radio waves or the like of a wirelessnetwork such as a mobile phone.

Far-end signals inputted to the far-end signal input terminals 101 a and101 b are outputted to the DA converters 102 a and 102 b. The DAconverters 102 a and 102 b convert the far-end signals from digitalsound signals to analog sound signals. Subsequently, the signals areoutputted to a near-end side via the loudspeakers 103 a and 103 b.

On the other hand, the microphones 104 receive analog sound signalssuperimposed with the talker's speech at the near end, environmentalsounds at the near-end, and acoustic echo signals (for example, analogsound signals outputted from the loudspeakers 103 a and 103 b,transferring in the space at the near-end side, and inputting). The ADconverter 105 converts the analog sound signals into digital soundsignals. Subsequently, the digital sound signals are inputted to thestereo echo suppressing device 100 as near-end input signals.

The far-end signal frequency domain transform units 106 a and 106 btransform the far-end signals in a time domain into spectrums in afrequency domain by using the FFT or the like, and outputs frequencyspectrums ROUTa(i,ω) and ROUTb(i,ω) of the transformed far-end signalsto the far-end signal amplitude spectrum calculating units 107 a and 107b.

The far-end signal amplitude spectrum calculating units 107 a and 107 bcalculate amplitude spectrums |ROUTa(i,ω)| and |ROUTb(i,ω)| of thefar-end signals in accordance with Equations (1) and (2) using thefrequency spectrums ROUTa(i,ω) and ROUTb(i,ω).

|ROUTa(i,Ψ)|=√{square root over((ROUTa_real(i,ω))²+(ROUTa)_image(i,ω))²)}{square root over((ROUTa_real(i,ω))²+(ROUTa)_image(i,ω))²)}{square root over((ROUTa_real(i,ω))²+(ROUTa)_image(i,ω))²)}  (1)

|ROUTb(i,ω)|=√{square root over((ROUTb_real(i,ω))²+(ROUTb)_image(i,ω))²)}{square root over((ROUTb_real(i,ω))²+(ROUTb)_image(i,ω))²)}{square root over((ROUTb_real(i,ω))²+(ROUTb)_image(i,ω))²)}  (2)

Where i represents a frame, ω represents a frequency bin,ROUTa_real(i,ω) and ROUTa_image(i,ω) represent a real part and animaginary part in the frequency spectrum ROUTa(i,ω) of a far-end signalof the frequency bin ω in the frame i, ROUTb_real(i,ω) andROUTb_image(i,ω) represent a real part and an imaginary part in thefrequency spectrum ROUTb(i,ω) of the far-end signal of the frequency binω in the frame i, and the frequency spectrums ROUTa(i,ω) and ROUTb(i,ω)of the far-end signal are represented by Equations (3) and (4).

ROUTa(i,ω)=ROUTa_real(i,ω)+j·ROUTa_image(i,ω)  (3)

ROUTb(i,ω)=ROUTb_real(i,ω)+j·ROUTb_image(i,ω)  (4)

Where j in Equations (3) and (4) represents an imaginary number. Thefrequency spectrums |ROUTa(i,ω)| and |ROUTb(i,ω)| of the far-end signalscalculated by the far-end signal amplitude spectrum calculating units107 a and 107 b are outputted to the estimation echo signal calculatingunits 109 a and 109 b and the echo path characteristic calculating units117 a and 117 b.

The estimation echo signal calculating units 109 a and 109 b calculateamplitude spectrums |ECHOa(i,ω)| and |ECHOb(i,ω)| of the estimation echosignals in accordance with Equations (5) and (6) using the echo pathcharacteristics |Ha(i−1,ω)| and |Hb(i−1,ω)| stored in the echo pathcharacteristic storage units 108 a and 108 b and the amplitude spectrums|ROUTa(i,ω)| and |ROUTb(i,ω)| of the far-end signals.

|ECHOa(i,ω)|=|Ha(i−1,ω)|·|ROUTa(i,ω)|  (5)

|ECHOb(i,ω)|=|Hb(i−1,ω)|·|ROUTb(i,ω)  (6)

In Equations (5) an (6), the amplitude spectrums |ROUTa(i,ω)| and|ROUTb(i,ω)| of the far-end signals are multiplied by frequency binscorresponding to the echo path characteristics |Ha(i−1,ω)| and|Hb(i−1,ω)| stored in the echo path characteristic storage unit 110, toobtain the amplitude spectrums |ECHOa(i,ω)| and |ECHOb(i,ω)| ofestimation echo signals of the frequency bins. Subsequently, theamplitude spectrum |ECHOa(i,ω)| of the estimation echo signal obtainedby the estimation echo signal calculating unit 109 a is outputted to theecho suppression gain calculating unit 113 a and the correlationcomponent rejecting unit 110. The amplitude spectrum |ECHOb(i,ω)| of theestimation echo signal obtained by the estimation echo signalcalculating unit 109 b is outputted to the correlation componentrejecting unit 110.

In accordance with Equation (7), the correlation component rejectingunit 110 calculates an amplitude spectrum |ECHO_corr_rejection(i,ω)|from which a correlation component between |ECHOa(i,ω)| and |ECHOb(i,ω)|included in |ECHOb(i,ω)| has been rejected.

                                           (7)${{{ECHO\_ corr}{\_ rejection}( {i,\omega} )}} = \{ \begin{matrix}{{{{ECHOb}( {i,\omega} )}} - {{{ECHOa}( {i,\omega} )}}} & ( {{{{{ECHOb}( {i,\omega} )}} - {{{ECHOa}( {i,\omega} )}}} > 0} ) \\0 & ({otherwise})\end{matrix} $

The correlation component rejecting unit 110 outputs the amplitudespectrum |ECHO_corr_rejection(i,ω)| from which the correlation componenthas been rejected, to the echo suppression gain calculating unit 113 b.

On the other hand, the near-end input signal frequency domain transformunit 111 transforms the near-end input signal in a time domain into aspectrum in a frequency domain by using the FFT or the like, and outputsa frequency spectrum SIN(i,ω) of the transformed near-end input signalto the near-end input signal amplitude spectrum calculating unit 112 andthe echo suppression processing unit 114 a.

The near-end input signal amplitude spectrum calculating unit 112calculates an amplitude spectrum |SIN(i,ω)| of the near-end input signalin accordance with Equation (8) using the frequency spectrum SIN(i,ω) ofthe near-end input signal.

|SIN(i,ω)|=√{square root over((SIN_real(i,ω))²+(SIN_image(i,ω))²)}{square root over((SIN_real(i,ω))²+(SIN_image(i,ω))²)}  (8)

Where SIN_real(i,ω) and SIN_image(i,ω) represent a real part and animaginary part in the frequency spectrum of the near-end input signal ofthe frequency bin ω in the frame i, and the frequency spectrum SIN(i,ω)of the near-end input signal is represented by Equation (9).

SIN(i,ω)=SIN_real(i,ω)+j·SIN_image(i,ω)  (9)

Where j in Equation (9) represents an imaginary number. Subsequently,the amplitude spectrum |SIN(i,ω)| of the near-end input signalcalculated by the near-end input signal amplitude spectrum calculatingunit 112 is outputted to the echo suppression gain calculating units 113a and 113 b and the echo path characteristic calculating units 117 a and117 b.

The echo suppression gain calculating unit 113 a acquires the amplitudespectrum |SIN(i,ω)| of the near-end input signal and the amplitudespectrum |ECHOa(i,ω)| of the estimation echo signal, and calculates anecho suppression gain Ga(i,ω) by using Equation (10).

$\begin{matrix}{{{Ga}( {i,\omega} )} = \frac{{{{SIN}( {i,\omega} )}} - {{{ECHOa}( {i,\omega} )}}}{{{SIN}( {i,\omega} )}}} & (10)\end{matrix}$

For each frequency bin, Equation (10) divides, by the amplitude spectrum|SIN(i,ω)| of the near-end input signal, an amplitude spectrum obtainedby subtracting the amplitude spectrum |ECHOa(i,ω)| of the estimationecho signal from the amplitude spectrum |SIN(i,ω)| of the near-end inputsignal, to obtain the echo suppression gain Ga(i,ω). The echosuppression gain Ga(i,ω) obtained by the echo suppression gaincalculating unit 113 a is outputted to the echo suppression processingunit 114 a.

The echo suppression gain calculating unit 113 b acquires the amplitudespectrum |SIN(i,ω)| of the near-end input signal and the amplitudespectrum |ECHO_corr_rejection(i,ω)| of the estimation echo signal fromwhich the correlation component between |ECHOa(i,ω)| and |ECHOb(i,ω)|included in |ECHOb(i,ω)| has been rejected, and calculates an echosuppression gain Gb(i,ω) by using Equation (11).

$\begin{matrix}{{{Gb}( {i,\omega} )} = \frac{{{{SIN}( {i,\omega} )}} - {{{ECHO\_ corr}{\_ rejection}( {i,\omega} )}}}{{{SIN}( {i,\omega} )}}} & (11)\end{matrix}$

For each frequency bin, Equation (11) divides, by the amplitude spectrum|SIN(i,ω)| of the near-end input signal, an amplitude spectrum obtainedby subtracting the amplitude spectrum |ECHO_corr_rejection(i,ω)| of theestimation echo signal from the amplitude spectrum |SIN(i,ω)| of thenear-end input signal, to obtain the echo suppression gain Gb(i,ω) whichthe correlation component between |ECHOa(i,ω)| and |ECHOb(i,ω)| includedin |ECHOb(i,ω)| has been rejected. The echo suppression gain Gb(i,ω)obtained by the echo suppression gain calculating unit 113 b isoutputted to the echo suppression processing unit 114 b.

The echo suppression processing unit 114 a suppresses an acoustic echosignal superimposed on the spectrum SIN(i,ω) of the near-end inputsignal in accordance with Equations (12) and (13) using the spectrumSIN(i,ω) of the near-end input signal and the echo suppression gainGa(i,ω).

SOUTa_real(i,ω)=Ga(i,ω)·SIN_real(i,ω)  (12)

SOUTa_image(i,ω)=Ga(i,ω)·SIN_image(i,ω)  (13)

Where SOUTa_real(i,ω) and SOUTa_image(i,ω) represent a real part and animaginary part in the frequency spectrum of the near-end output signalof the frequency bin ω in the frame i, and the frequency spectrumSOUTa(i,ω) of the near-end output signal is represented by an Equation(14) as follows.

SOUTa(i,ω)=SOUTa_real(i,ω)+j·SOUTa_image(i,ω)  (14)

Where j in Equation (14) represents an imaginary number. Equation (12)and Equation (13) multiply a real part and an imaginary part in thefrequency spectrum by the echo suppression gain Ga(i,ω) for eachfrequency bin, to obtain a frequency spectrum SOUTa(i,ω) of the near-endoutput signal suppressing the acoustic echo signal. Subsequently, thefrequency spectrum SOUTa(i,ω) of the near-end output signal suppressingthe acoustic echo signal obtained by the echo suppression processingunit 114 a is outputted to the echo suppression processing unit 114 b.

The echo suppression processing unit 114 b suppresses an acoustic echosignal superimposed on the spectrum SINb(i,ω) of the near-end inputsignal in accordance with Equations (15) and (16) using the spectrumSOUTa(i,ω) of the near-end output signal and the echo suppression gainGb(i,ω).

SOUTb_real(i,ω)=Gb(i,ω)·SOUTa_real(i,ω)  (15)

SOUTb_image(i,ω)=Gb(i,ω)·SOUTa_image(i,ω)  (16)

Where SOUTb_real(i,ω) and SOUTb_image(i,ω) represent a real part and animaginary part in the frequency spectrum of the near-end output signalof the frequency bin ω in the frame i, and the frequency spectrumSOUTb(i,ω) of the near-end output signal is represented by an Equation(17) as follows.

SOUTb(i,ω)=SOUTb_real(i,ω)+j·SOUTb_image(i,ω)  (17)

Where j in Equation (17) represents an imaginary number. Equation (15)and Equation (16) multiply a real part and an imaginary part in thefrequency spectrum by the echo suppression gain Gb(i,ω) for eachfrequency bin, to obtain a frequency spectrum SOUTb(i,ω) of the near-endoutput signal suppressing the acoustic echo signal. Subsequently, thefrequency spectrum SOUTb(i,ω) of the near-end output signal suppressingthe acoustic echo signal obtained by the echo suppression processingunit 114 b is outputted to the near-end output signal time domaintransform unit 115.

The near-end output signal time domain transform unit 115 transforms thefrequency spectrum SOUTb(i,ω) of the near-end output signal in afrequency domain into a digital sound signal in a time domain by usingan IFFT or the like, and outputs the near-end output signal to thenear-end signal output terminal 116 a.

The near-end signal output terminal 116 a is connected to, for example,a network such as an IP network or radio waves or the like of a wirelessnetwork such as a mobile phone. Via a connected line, the near-endoutput signal is outputted to the far-end side that is a call partner.

The echo path characteristic calculating units 117 a and 117 b calculateecho path characteristics |Ha1(i,ω)| and |Hb1(i,ω)| of the current frameby using the amplitude spectrums |ROUTa(i,ω)| and |ROUTb(i,ω)| of thefar-end signals from the far-end signal amplitude spectrum calculatingunit 107 a and 107 b and the amplitude spectrum |SIN(i,ω)| of thenear-end output signal from the near-end signal amplitude spectrumcalculating unit 112.

$\begin{matrix}{{{{Ha}_{1}( {i,\omega} )}} = \frac{{{SIN}( {i,\omega} )}}{{{ROUTa}( {i,\omega} )}}} & (18) \\{{{{Hb}_{1}( {i,\omega} )}} = \frac{{{SIN}( {i,\omega} )}}{{{ROUTb}( {i,\omega} )}}} & (19)\end{matrix}$

When the echo path characteristics |Ha1(i,ω)| and |Hb1(i,ω)| of thecurrent frame are obtained, the echo path characteristics |Ha1(i,ω)| and|Hb1(i,ω)| of the current frame are outputted to the echo pathcharacteristic updating units 118 a and 118 b.

The echo path characteristic updating units 118 a and 118 b read out theecho path characteristics |Ha1(i,ω)| and |Hb1(i,ω)| from the echo pathcharacteristic calculating units 117 a and 117 b and the echo pathcharacteristics |Ha(i−1,ω)| and |Hb(i−1,ω)| stored in the echo pathcharacteristic storage units 108 a and 108 b, and update the echo pathcharacteristics in accordance with Equation (20) and Equation (21) using|Ha(i−1,ω)| and |Ha1(i,ω)|, and |Hb(i−1,ω)| and |Hb1 (i,ω)|.

|Ha(i,ω)|=(1−c)·|Ha ₁(i,ω)|+c·|Ha(i−1,ω)|  (20)

|Hb(i,ω)|=(1−c)·|Hb ₁(i,ω)|+c·|Hb(i−1,ω)|  (21)

Where c represents a coefficient of a time constant filter, and c is avalue which is 0 or larger and 1 or smaller. In a case in which a userwants to slow the update of the echo path characteristics, it isdesirable that c is close to 1 (for example, a value such as c=0.99). Ina case in which a user wants to quicken the update, it is desirable thatc is close to 0 (for example, a value such as c=0.01). The echo pathcharacteristic updating units 118 a and 118 b output the updated echopath characteristics to the echo path characteristic storage units 108 aand 108 b and the echo path characteristic storage units 108 a and 108 bstore the updated echo path characteristics.

(A-3) Effect of First Embodiment

As described above, according to the first embodiment, even when thecorrelated signals are inputted as far-end signals, distortion in soundsignals due to too much subtraction can be prevented by rejecting acorrelation component of the estimation echo signal, and an acousticecho signal can be suppressed.

According to the first embodiment, the stereo echo suppression processis performed in a frequency domain without using the adaptablealgorithm. Since the adaptable algorithm is not used in this way, theacoustic echo signal can be suppressed while there is no coefficientindeterminacy problem. In addition, according to the first embodiment,in the case of a signal in which a Left channel and a Right channel of afar-end signal are correlated, it is possible to prevent too muchsuppression while suppressing the acoustic echo signal in view of thecorrelation of the estimation echo signal estimated in the stereo echosuppression process.

(B) Second Embodiment

Next, with reference to the drawings, a stereo echo suppressing device,stereo echo suppressing method, and stereo echo suppressing programaccording to a second embodiment of the present invention are explained.

In the second embodiment, the present embodiment is also applied to astereo echo suppressing device, method, and program, the stereo echosuppressing device being mounted on a voice transmitting and receivingdevice in the amplifying call system such as the teleconferencing systemand the audio conferencing system.

(B-1) Configuration in Second Embodiment

FIG. 2 is a block diagram showing a configuration of an echo suppressingdevice 200 according to the second embodiment. With reference to FIG. 2,the echo suppressing device 200 according to the second embodimentincludes the far-end signal input terminals 101 a and 101 b, the DAconverters 102 a and 102 b, the loudspeakers 103 a and 103 b, themicrophones 104 a and 104 b, the AD converters 105 a and 105 b, stereoecho suppressing devices 200 a and 200 b, and the near-end signal outputterminals 116 a and 116 b.

The stereo echo suppressing devices 200 a and 200 b each include thefar-end signal frequency domain transform units 106 a and 106 b, thefar-end signal amplitude spectrum calculating units 107 a and 107 b, theecho path characteristic storage units 108 a and 108 b, the estimationecho signal calculating units 109 a and 109 b, the correlationcalculating unit 201, the near-end input signal frequency domaintransform unit 111, the near-end input signal amplitude spectrumcalculating unit 112, the echo suppression gain calculating unit 113 a,the correlation echo suppression gain calculating unit 202, the echosuppression processing units 114 a and 114 b, the near-end output signaltime domain transform unit 115, the echo path characteristic calculatingunits 117 a and 117 b, and the echo path characteristic updating units118 a and 118 b.

Note that, the “first echo suppressing unit” described in claimsincludes the echo suppression gain calculating unit 113 a and the echosuppression processing unit 114 a, and the “second echo suppressingunit” described in claims includes the correlation calculating unit 201,the correlation echo suppression gain calculating unit 202 and the echosuppression processing unit 114 b.

The second embodiment is different from the echo suppressing device 100according to the first embodiment in a calculation way of a correlationof an estimation echo signal and an echo suppression gain. The otherstructural elements according to the second embodiments are similar toor the same as the structural elements of the echo suppressing device100 in FIG. 1 according to the first embodiment.

In other words, stereo echo suppressing devices 200 a and 200 b haveconfigurations different from the stereo echo suppressing devices 100 aand 100 b according to the first embodiment, since the stereo echosuppressing devices 200 a and 200 b each include the correlationcalculating unit 201 instead of the correlation component rejecting unit110 explained in the first embodiment, and includes the correlation echosuppression gain calculating unit 202 instead of the echo suppressiongain calculating unit 113 b.

In FIG. 2, structural elements that have same structure or structurecorresponding to the structural elements of the echo suppressing device100 according to the first embodiment are denoted with the samereference numerals. Note that, a repeated explanation of structuralelements that are the same as or corresponding to those of the firstembodiment is omitted.

Note that, the stereo echo suppressing devices 200 a and 200 b have thesame configurations also in the second embodiment. Accordingly,hereinafter, configuration of only the stereo echo suppressing device200 a is explained.

The correlation calculating unit 201 calculates a correlation between anamplitude spectrum of an estimation echo signal outputted from theestimation echo signal calculating unit 109 a and an amplitude spectrumof an estimation echo signal outputted from the estimation echo signalcalculating unit 109 b. The correlation calculating unit 201 outputs acalculation result of the correlation to the correlation echosuppression gain calculating unit 202.

The correlation echo suppression gain calculating unit 202 calculates anecho suppression gain for suppressing the acoustic echo signalsuperimposed on the near-end input signal, by using the amplitudespectrum of the near-end input signal from the near-end input signalamplitude spectrum calculating unit 112, the amplitude spectrum of theestimation echo signal from the estimation echo signal calculating unit109 b, and the calculation result of the correlation by the correlationcalculating unit 201. The correlation echo suppression gain calculatingunit 202 outputs the calculated echo suppression gain to the echosuppression processing unit 114 b.

(B-2) Operation in Second Embodiment

Next, details of operation of echo suppression process in the stereoecho suppressing device 200 according to the second embodiment of thepresent invention are explained. Here, details of processing operationsof the correlation calculating unit 201 and the correlation echosuppression gain calculating unit 202 that are included in the stereoecho suppressing device 200 a according to the second embodiment areexplained.

For example, in accordance with Equation (22), the correlationcalculating unit 201 in the stereo echo suppressing device 200calculates a correlation between amplitude spectrums |ECHOa(i,ω)| and|ECHOb(i,ω)| of the estimation echo signal outputted from the estimationecho signal calculating units 109 a and 109 b.

$\begin{matrix}{{{Corr}(i)} = \frac{\begin{matrix}{\sum\limits_{k = 0}^{{FFT\_ LEN}/2}( {{{{ECHOa}( {i,k} )}} - {aveECHOa}} )} \\( {{{{ECHOb}( {i,k} )}} - {aveECHOb}} )\end{matrix}}{\begin{matrix}{\sqrt{\sum\limits_{k = 0}^{{FFT\_ LEN}/2}( {{{{ECHOa}( {i,k} )}} - {aveECHOa}} )^{2}} \cdot} \\\sqrt{\sum\limits_{k = 0}^{{FFT\_ LEN}/2}( {{{{ECHOb}( {i,k} )}} - {aveECHOb}} )^{2}}\end{matrix}}} & (22)\end{matrix}$

In Equation (22), Corr(i) represents a correlation coefficient, andFFT_LEN represents FFT length. aveECHOa represents a frame average valueof an amplitude spectrum of the estimation echo signal |ECHOa(i,ω)|, andaveECHOb represents a frame average value of the amplitude spectrum ofthe estimation echo signal |ECHOb(i,ω)|. For example, aveECHOa andaveECHOb can be obtained in accordance with Equation (23) and Equation(24), respectively.

$\begin{matrix}{{aveECHOa} = \frac{\sum\limits_{k = 0}^{{FFT\_ LEN}/2}{{{ECHOa}( {i,k} )}}}{( {{FFT\_ LEN}/2} )}} & (23) \\{{aveECHOb} = \frac{\sum\limits_{k = 0}^{{FFT\_ LEN}/2}{{{ECHOb}( {i,k} )}}}{( {{FFT\_ LEN}/2} )}} & (24)\end{matrix}$

After the correlation calculating unit 201 calculates a correlationcoefficient Corr(i), the correlation coefficient Corr(i) is outputted tothe correlation echo suppression gain calculating unit 202.

To the calculation way of a correlation performed by the correlationcalculating unit 201, diverse methods can be applied. For example, thecorrelation calculating unit 201 may calculate a difference between theamplitude spectrums |ECHOa(i,ω)| and |ECHOb(i,ω)| of the estimation echosignal of the far-end signals for each frequency bin, and output a sumof the differences as the correlation coefficient Corr(i) to thecorrelation echo suppression gain calculating unit 202. In this case,when the sum of the differences is small, it is determined that there isa correlation. When the sum of the differences is large, it isdetermined that there is no correlation.

The correlation echo suppression gain calculating unit 202 acquires theamplitude spectrum |SIN(i,ω)| of the near-end input signal, theamplitude spectrum |ECHOb(i,ω)| of the estimation echo signal, and thecorrelation coefficient Corr(i), and calculates an echo suppression gainGb(i,ω) by using Equations (25).

$\begin{matrix}{{{Gb}( {i,\omega} )} = \{ \begin{matrix}\frac{{{{SIN}( {i,\omega} )}} - {{{ECHOb}( {i,\omega} )}}}{{{SIN}( {i,\omega} )}} & ( {{- {TH}} < {{Corr}(i)} < {TH}}  \\{gain} & ({otherwise})\end{matrix} } & (25)\end{matrix}$

Where TH represents a threshold and gain represents an echo suppressiongain when there is a correlation. For each frequency bin, the upperequation of Equations (25) divides, by the amplitude spectrum |SIN(i,ω)|of the near-end input signal, an amplitude spectrum obtained bysubtracting the amplitude spectrum |ECHOb(i,ω)| of the estimation echosignal from the amplitude spectrum |SIN(i,ω)| of the near-end inputsignal only in a case of a frame determined as no correlation from thecorrelation coefficient Corr(i), so as to obtain the echo suppressiongain Gb(i,ω). In the second embodiment, the correlation coefficientCorr(i) is a value between −1 and 1. When Corr(i) is close to 1, thereis a positive correlation. When Corr(i) is close to −1, there is anegative correlation. In the case when Corr(i) is close to 0, there isno correlation, then the echo suppression gain Gb(i,ω) represented bythe upper Equation of Equations (25). For example, when TH=0.8 andgain=1.0, the echo suppression gain is 1.0 when there is a positivecorrelation or a negative correlation. The echo suppression gain Gb(i,ω)calculated by the echo suppression gain calculating unit 113 b isoutputted to the echo suppression processing unit 114 b.

(B-3) Effect of Second Embodiment

As described above, according to the second embodiment, even when thecorrelated signals are inputted as far-end signals, in the case that tis determined that there is a correlation, too much subtraction does notoccur in a second subtraction after the correlation of the estimationecho signal is calculated, it is possible to prevent distortion in soundsignals due to too much subtraction while suppressing an acoustic echosignal.

(C) Another Embodiment

The diverse modified embodiments have been explained in theabove-described embodiments. In addition, the present invention can beapplied to a modified embodiment as follows.

(C-1) The case in which the echo suppressing device according to each ofthe above-described embodiments includes the two microphones has beenexplained. In addition, more microphones can be installed in the echosuppressing device according to each of the above-described embodiments.

For example, FIG. 3 is a block diagram showing a configuration of anecho suppressing device 100A in a case in which more microphones areextended in the echo suppressing device according to the firstembodiment. Meanwhile, FIG. 4 is a block diagram showing a configurationof an echo suppressing device 200A in a case in which more microphonesare extended in the echo suppressing device according to the secondembodiment.

With reference to FIGS. 3 and 4, as explained in the first and secondembodiments, the echo suppressing device 100A (200A) receives far-endsignals that are stereo signals of two channels (Left channel and Rightchannel), and includes stereo echo suppressing devices 100 a, 100 b, . .. , 100 n (200 a, 200 b, . . . , 200 n) for respective microphones 104a, 104 b, . . . , 104 n. To configurations and processing operations ofthe stereo echo suppressing devices 100 a, 100 b, . . . , 100 n (200 a,200 b, . . . , 200 n), the configurations and the processing operationsexplained in the first and second embodiment can be applied,respectively. Therefore, detailed explanations are omitted here. Theeffects similar to the first and second embodiment can be obtained alsoin the case of multi-channeled microphone input since stereo echosuppressing devices are provided in accordance with the extension ofmore microphones. A microphone extension condition is that moremicrophones can be extended under a processing limit of a processingdevice such as an implemented dedicated board, DSP, and CPU.

(C-2) For example, the echo suppressing device explained in each of theabove-described embodiments may be mounted on a device including a voicecommunication device used for the teleconferencing system, the audioconferencing system, or the like. In addition, the echo suppressingdevice according to the embodiments of the present invention may bemounted on a mobile terminal such as a mobile phone or a smartphone.

Note that the stereo echo suppressing method of the embodimentsdescribed above can be configured as the stereo echo suppressingprogram. In the case of the stereo echo suppressing program, the programthat implements at least part of the stereo echo suppressing method maybe stored in a non-transitory computer readable medium, such as aflexible disk or a CD-ROM, and may be loaded onto a computer andexecuted. The recording medium is not limited to a removable recordingmedium such as a magnetic disk or an optical disk, and may be a fixedrecording medium such as a hard disk apparatus or a memory. In addition,the program that implements at least part of the stereo echo suppressingmethod may be distributed through a communication line (also includingwireless communication) such as the Internet. Furthermore, the programmay be encrypted or modulated or compressed, and the resulting programmay be distributed through a wired or wireless line such as theInternet, or may be stored a non-transitory computer readable medium anddistributed.

Heretofore, preferred embodiments of the present invention have beendescribed in detail with reference to the appended drawings, but thepresent invention is not limited thereto. It should be understood bythose skilled in the art that various changes and alterations may bemade without departing from the spirit and scope of the appended claims.

What is claimed is:
 1. A stereo echo suppressing device that suppressesan acoustic echo signal based on a near-end input signal and far-endsignals of two channels as a stereo signal, the stereo echo suppressingdevice comprising: a first far-end signal amplitude spectrum calculatingunit configured to calculate an amplitude spectrum of a first far-endsignal by transforming the inputted first far-end signal into a spectrumin a frequency domain; a second far-end signal amplitude spectrumcalculating unit configured to calculate an amplitude spectrum of asecond far-end signal by transforming the inputted second far-end signalinto a spectrum in a frequency domain; a near-end input signal amplitudespectrum calculating unit configured to calculate an amplitude spectrumof the near-end input signal by transforming the inputted near-end inputsignal into a spectrum in a frequency domain; a first estimation echosignal estimating unit configured to estimate an amplitude spectrum of afirst estimation echo signal by multiplying a stored first echo pathcharacteristic by the amplitude spectrum of the first far-end signal; asecond estimation echo signal estimating unit configured to estimate anamplitude spectrum of a second estimation echo signal by multiplying astored second echo path characteristic by the amplitude spectrum of thesecond far-end signal; a first echo suppressing unit configured tosuppress the acoustic echo signal superimposed on the near-end inputsignal by calculating a first echo suppression gain with the amplitudespectrum of the first estimation echo signal and the amplitude spectrumof the near-end input signal; a second echo suppressing unit configuredto suppress the acoustic echo signal superimposed on the near-end inputsignal by calculating a second echo suppression gain according to acorrelation result between the amplitude spectrum of the firstestimation echo signal and the amplitude spectrum of the secondestimation echo signal; a first echo path characteristic updating unitconfigured to calculate and update the echo path characteristic of thefirst far-end signal by using the amplitude spectrum of the firstfar-end signal and the amplitude spectrum of the near-end input signal;and a second echo path characteristic updating unit configured tocalculate and update the echo path characteristic of the second far-endsignal by using the amplitude spectrum of the second far-end signal andthe amplitude spectrum of the near-end input signal.
 2. The stereo echosuppressing device according to claim 1, wherein the second echosuppressing unit includes: a correlation component rejecting unitconfigured to reject a correlation component between the amplitudespectrum of the second estimation echo signal and the amplitude spectrumof the first estimation echo signal, at each frequency; an echosuppression gain calculating unit configured to calculate a second echosuppression gain by using the amplitude spectrum of the near-end inputsignal and the amplitude spectrums of the estimation echo signals fromwhich the correlation component has been rejected by the correlationcomponent rejecting unit; and an echo suppression processing unitconfigured to suppress the acoustic echo signal to be superimposed onthe near-end input signal, by using the second echo suppression gain. 3.The stereo echo suppressing device according to claim 2, wherein thecorrelation component rejecting unit rejects the correlation componentby using a difference between the amplitude spectrum of the secondestimation echo signal and the amplitude spectrum of the firstestimation echo signal.
 4. The stereo echo suppressing device accordingto claim 1, wherein the second echo suppressing unit includes acorrelation calculating unit configured to calculate a correlationcoefficient of the amplitude spectrums of the estimation echo signals byusing the amplitude spectrum of the first estimation echo signal and theamplitude spectrum of the second estimation echo signal, an echosuppression gain calculating unit configured to calculate a second echosuppression gain on the basis of the correlation coefficient of theamplitude spectrums of the estimation echo signals obtained by thecorrelation calculating unit, and an echo suppression processing unitconfigured to suppress the acoustic echo signal to be superimposed onthe near-end input signal, by using the second echo suppression gain. 5.An echo suppressing device comprising: one or a plurality of the stereoecho suppressing device according to claim 1, wherein the respectivenear-end input signals are inputted from one or a plurality ofmicrophones.
 6. A stereo echo suppressing method of suppressing anacoustic echo signal based on a near-end input signal and far-endsignals of two channels as a stereo signal, the stereo echo suppressingmethod comprising: calculating an amplitude spectrum of a first far-endsignal by a first far-end signal amplitude spectrum calculating unit, bytransforming the inputted first far-end signal into a spectrum in afrequency domain; calculating an amplitude spectrum of a second far-endsignal by a second far-end signal amplitude spectrum calculating unit,by transforming the inputted second far-end signal into a spectrum in afrequency domain; calculating an amplitude spectrum of the near-endinput signal by a near-end input signal amplitude spectrum calculatingunit, by transforming the inputted near-end input signal into a spectrumin a frequency domain; estimating an amplitude spectrum of a firstestimation echo signal by a first estimation echo signal estimatingunit, by multiplying a stored echo path characteristic of the firstfar-end signal by the amplitude spectrum of the first far-end signal;estimating an amplitude spectrum of a second estimation echo signal by asecond estimation echo signal estimating unit, by multiplying a storedecho path characteristic of the second far-end signal by the amplitudespectrum of the second far-end signal; suppressing the acoustic echosignal superimposed on the near-end input signal by a first echosuppressing unit, by calculating a first echo suppression gain by usingthe amplitude spectrum of the first estimation echo signal and theamplitude spectrum of the near-end input signal; suppressing theacoustic echo signal superimposed on the near-end input signal by asecond echo suppressing unit, by calculating a second echo suppressiongain according to a correlation result between the amplitude spectrum ofthe first estimation echo signal and the amplitude spectrum of thesecond estimation echo signal; calculating and updating the echo pathcharacteristic of the first far-end signal by a first echo pathcharacteristic updating unit, by using the amplitude spectrum of thefirst far-end signal and the amplitude spectrum of the near-end inputsignal; and calculating and updating the echo path characteristic of thesecond far-end signal by a second echo path characteristic updatingunit, by using the amplitude spectrum of the second far-end signal andthe amplitude spectrum of the near-end input signal.
 7. A non-transitorycomputer-readable recording medium storing a stereo echo suppressingprogram that suppresses an acoustic echo signal based on a near-endinput signal and far-end signals of two channels as a stereo signal, thestereo echo suppressing program causing a computer to function as: afirst far-end signal amplitude spectrum calculating unit configured tocalculate an amplitude spectrum of a first far-end signal bytransforming the inputted first far-end signal into a spectrum in afrequency domain; a second far-end signal amplitude spectrum calculatingunit configured to calculate an amplitude spectrum of a second far-endsignal by transforming the inputted second far-end signal into aspectrum in a frequency domain; a near-end input signal amplitudespectrum calculating unit configured to calculate an amplitude spectrumof the near-end input signal by transforming an inputted near-end inputsignal into a spectrum in a frequency domain; a first estimation echosignal estimating unit configured to calculate an amplitude spectrum ofa first estimation echo signal by multiplying a stored echo pathcharacteristic of the first far-end signal by the amplitude spectrum ofthe first far-end signal; a second estimation echo signal estimatingunit configured to calculate an amplitude spectrum of a secondestimation echo signal by multiplying a stored echo path characteristicof the second far-end signal by the amplitude spectrum of the secondfar-end signal; a first echo suppressing unit configured to suppress theacoustic echo signal superimposed on the near-end input signal bycalculating a first echo suppression gain by using the amplitudespectrum of the first estimation echo signal and the amplitude spectrumof the near-end input signal; a second echo suppressing unit configuredto suppress the acoustic echo signal superimposed on the near-end inputsignal by calculating a second echo suppression gain according to acorrelation result between the amplitude spectrum of the firstestimation echo signal and the amplitude spectrum of the secondestimation echo signal; a first echo path characteristic updating unitconfigured to calculate and update the echo path characteristic of thefirst far-end signal by using the amplitude spectrum of the firstfar-end signal and the amplitude spectrum of the near-end input signal;and a second echo path characteristic updating unit configured tocalculate and update the echo path characteristic of the second far-endsignal by using the amplitude spectrum of the second far-end signal andthe amplitude spectrum of the near-end input signal.